Sadly, yes.
An actual game that I'm working have this feature, and combined with 'downloads in-game', built a social environment between players of the fangame, and I wanted to see this expanded between other games, because Essentials have more users.
Sadly, PSP DS is hard to use, and seeing that this feature is not to hard to code in like you said, why no one made it yet?

If this feature is not hard to code in, by all means do it. One game using Essentials had an "offline" trade/battle system - Pokemon Raptor. It used .txt files for the transfer of data, and trade confirmations. We are trying to avoid that, and create a system that would do everything automatically for you. Having online connectivity, on a global server, removes this need for manual connections/transfers and possible screw-ups.

EDIT:

Quote originally posted by Maruno:

It would be more secure to have the battle worked out by a host, but I don't think there'd be any risk in skimping on that in favour of convenience (faster, less information to transmit, etc.). I can't think of anything else that would cause desync.